1. Field of the Invention
The present invention relates to a ball screw mechanism in which a nut member is screwed on a ball screw shaft through balls, and by which the nut member is displaced axially along the ball screw shaft under the driving action of a rotary drive source.
2. Description of the Related Art
Heretofore, a ball screw mechanism has been known, which is equipped with a screw shaft having helical grooves engraved on an outer circumferential surface thereof, a cylindrical shaped nut member disposed on the outer circumferential side of the screw shaft, and balls disposed via the helical grooves between the screw shaft and the nut member.
In such a ball screw mechanism, for example, a rotary drive source such as a motor or the like is connected with respect to the screw shaft, and by rotation of the screw shaft, the rotational motion of the screw is converted through the balls into linear motion of the nut member, whereby the nut member is displaced linearly along the axis of the screw shaft.
With the aforementioned ball screw mechanism, it is necessary for balls to be circulated between the nut member and the screw shaft accompanying displacement of the nut member in the axial direction. Owing thereto, for example, as in the ball screw mechanism disclosed in Japanese Laid-Open Patent Publication No. 2005-048845, end members are provided respectively on both ends of the nut member having circulation passages therein, and a return passage is disposed along the axial direction of the nut member interconnecting the end members. Then, when the nut member is displaced along the screw shaft, the balls mounted between the nut member and the screw shaft are circulated by being moved from the helical grooves on one end side of the nut member, through the circulation path in one of the end members, to the ball return passage, through the circulation path of the other end member, and once again the balls are moved into the helical grooves at the other end side of the nut member.
However, in the aforementioned conventional art, because a ball return passage through which the balls are circulated is disposed on an outer circumferential side of a hole through which the screw shaft is inserted through the nut member, it is necessary that the nut member be diametrically expanded in a radial outward direction, by an amount corresponding to the portion of the ball return passage. As a result, the nut member becomes larger in size.
Further, the process steps for forming the ball return passage, for returning the balls along the axial direction with respect to the nut member, are troublesome. Also, because the end members are constructed so as to be installed inside both ends of the nut member, the shape of the end members and the shapes at the attachment regions of the end members in the nut member are complex. As a result, fabrication and assembly of the nut member is complicated and productivity of the ball screw mechanism is lowered.